1. Field of the Invention. This invention pertains to thermal spraying, and more particularly to improved guns for spraying metallic and ceramic particles onto a substrate.
2. Description of the Prior Art. Various equipment has been developed to coat a substrate made of a first material with a layer of a different material. Such equipment includes plasma arc spray guns, in which fine particulate matter is entrained in, heated, and accelerated by a plasma stream. The plasma stream is directed to the substrate such that the coating particles are deposited onto the substrate. Creation of the plasma stream is normally accomplished by an electric arc. The plasma stream may have subsonic or supersonic speeds. Typical examples of prior plasma arc spray guns may be seen in U.S. Pat. Nos. 3,740,522; 3,823,302; and 4,127,760.
A commercially available plasma arc spray gun is manufactured and marketed by Miller Thermal, Inc. of Appleton, Wis., under Model SG-100. In FIGS. 1 and 2, reference numeral 1 refers to a typical subsonic version of the Miller Thermal, Inc. Model SG-100 plasma arc spray gun. The plasma arc spray gun 1 includes a rear housing 3, a center housing 5, and a front housing 7. The rear housing 3, center housing 5, and front housing 7 are generally tubular in shape and have a common longitudinal axis 9. Suitable screws, not shown, connect the rear housing, center housing, and front housing together by means of longitudinally extending holes 11 in the center housing and cooperating threads, not shown, in the rear housing and counterbored holes, also not shown, in the front housing. A front cover 13 is attached to the front housing, as by screws, not shown, passing through counterbored holes 15 in the front cover 13.
Retained inside the rear housing 3 and the center housing 5 of the plasma arc spray gun 1 is a cathode holder 16, the back end of which is formed with a fitting 19. There is a groove 30 around the outer diameter of the cathode holder 16 that cooperates with an internal surface of the center housing to form a circumferential passage 31. The front end 23 of the cathode holder 16 is tapped to receive a cathode assembly 25. The cathode assembly 25 includes a tip 29 and a fitting section 106. There is a distinct step 104 between the outer surface 107 of the fitting section 106 and the adjacent outer surface 98 of the tip 29.
Located inside the center housing 5 and the front housing 7 of the plasma arc spray gun 1 is a tubular anode 33. The anode 33 has a longitudinal axis that is coaxial with the axis 9. The interior of the anode is divided into three sections. A front interior section 35 has a cylindrical inner surface 36. A middle interior section 37 has a frusto-conical surface 38 with a first included angle. A back interior section 39 has a frusto-conical inner surface 42 with a second included angle that is less than the first included angle. The tip 29 of the cathode assembly 25 is so dimensioned and located relative to the anode 33 that the tip end 40 is quite close to the junction 44 of the anode front and middle interior sections 35 and 37, respectively. Two radial holes 41 pass through the anode from the front interior section 35.
Sandwiched between the front end 23 of the cathode holder 16 and the back end 45 of the anode 33 is an injector ring 47. The outer diameter of the injector ring 47 cooperates with an internal surface of the center housing 5 to form an annular passage 53. Holes 55 through the injector ring lead between the annular passage 53 and an annular space 57 located between the inner diameter 59 of the injector ring and the outer surface 107 of the cathode assembly 25. The axial center lines of the holes 55 are usually generally tangential to the injector ring inner diameter 59. The diameter of the inner surface 42 of the back interior section 39 at the back end 45 of the anode 33 is larger than the inner diameter 59 of the injector ring 47. Consequently, a step 69 exists between the inner surface 42 of the anode and the inner diameter 59 of the injector ring.
A suitable hole not shown, in the rear housing 3 connects with a hole 58 in the center housing 5 and the annular passage 53. A fitting, not shown, is connected to the hole in the rear housing. The fitting is connected to a source of primary gas. Supplying the primary gas to the fitting causes the gas to flow into the annular passage 53, through the holes 55, and into the annular space 57. Because of the tangential nature of the holes 55 in the injector ring 47, the primary gas enters the annular space 57 with an angular velocity. From the annular space, the primary gas flows through the anode interior sections 39 and 37, around the tip 29 of the cathode assembly 25, through the anode front interior section 35, and out the plasma arc spray gun 1 through a hole 60 in the front cover 13. The circular velocity of the primary gas creates a vortex within the anode interior sections.
A fitting 62 is connected to a tapped radial hole 64 in the front housing 7. A ceramic or metallic powder is supplied via the fitting 62 to the anode front interior section 35 by means of the front housing hole 64 and one of the radial holes 41 in the anode 33. The powder is entrained in the primary gas stream as the gas flows through the anode interior section 35. The fitting 19 of the cathode holder 16 is connected to a sink for cooling water. A second water fitting 61 is brazed into a port 66 in the front housing 7. Suitable internal passages, not shown, in the front housing connect the port 66 to passages 63 and 65 in the center housing 5. The center housing passage 65 connects with the annular passage 31 and another passage 67 in the cathode holder 16. The passage 67 leads to an outlet port 68 in the fitting 19. In that manner, cooling water supplied to the fitting 61 passes through the various internal passages 66, 63, 65, 31, 67, and 68 to cool the plasma arc spray gun 1.
The fitting 19 of the cathode holder 16 and the water fitting 61 also serve as connectors for electrical cables, not shown. When electrical power is supplied to the plasma arc spray gun 1 through the fittings, an arc is created between the end 40 of the tip 29 of the cathode assembly 25 and the anode 33. Ideally, the point of contact of the arc with the anode moves circumferentially around the anode interior under the impetus of the angular velocity of the primary gas vortex. The arc heats the primary gas flowing past the cathode tip to create a plasma stream. The plasma stream heats the powder entering the anode front interior section 35 through the fitting 62 and accelerates the powder out the plasma arc spray gun 1 to be deposited onto a substrate in known manner. Typically, the deposition efficiency of the plasma arc spray gun is in the order of 50 percent.
Prior subsonic plasma arc spray guns 1 have been in commercial use for many years and have given countless hours of satisfactory service. On the other hand, they are subject to improvement. Specifically, it is desirable that their deposition efficiencies be increased above those presently attainable.
In addition, under some operating conditions the arc between the tip 29 of the cathode assembly 25 and the anode 33 tends to lock in at a specific point on the interior of the anode rather than to continuously travel circumferentially around the anode interior. The stationary arc causes the anode surface to pit. The result is a loss of performance of the plasma arc spray gun 1 to the extent that the anode must be replaced. A typical service life of prior anodes is approximately 40 hours. It is desirable to increase the anode service life.
A drawback of some prior plasma arc spray guns concerns the center housing, such as the center housing 5 of the plasma arc spray gun 1. The center housing is invariably manufactured from an electrically insulative material. In certain situations, the material can become dimensionally unstable. Atmospheric moisture and cooling water, among other influences, can cause the center housing to vary in size during operation. As a consequence, the primary gas that should enter the anode interior section 39 only through the annular space 57 and the holes 55 in the injector ring 47 actually leaks past the joints between the injector ring and the back end 45 of the anode 33 and the front end 23 of the cathode holder 16. The effect is an unstable plasma stream emitting from the outlet hole 60 of the plasma arc spray gun. The unstable plasma stream has detrimental effects on the spray process.